Speed of sound of an ultrasonic wave that propagates inside of a bone can be used for an index of diagnosing bone strength. Conventionally, devices have existed that measures the speed of sound of the ultrasonic wave that propagates along the surface of a long pipe-shaped bone (i.e., long bone) in the longitudinal direction to diagnose bone strength based on the speed of sound.
As devices for measuring the speed of sound of the ultrasonic wave propagating along the bone surface, as shown in FIG. 17A, a device 901 including a transducer 902 for wave transmission and two transducers 903 and 904 for wave reception is well-known. The device 901 measures the speed of sound by the following method assuming that the bone surface is flat, as well as the surface of soft tissues, such as muscles covering the bone, being parallel to each other.
First, the transducer 902 for wave transmission transmits an ultrasonic wave to be incident to the bone surface near a critical angle to generate a surface wave on the bone surface. The surface wave propagates along the bone surface while emitting a leaky surface wave at a predetermined angle (same angle as the critical angle). The leaky surface wave is received by the two transducers 903 and 904 for wave reception. Because the interval between the two transducers 903 and 904 for wave reception is known, the speed of sound of the surface wave can be calculated from a time difference between the times when the transducers 903 and 904 received the leaky surface wave, respectively.
However, the bone surface and the surface of soft tissues may not be parallel to each other and, thus, in that case, the device 901 may produce errors in its calculation results. There are various devices proposed to eliminate the errors (for example, refer to JP2003-517328(A), WO03/099132, and WO03/099133).
As one particular example of such devices, as shown in FIG. 17B, a speed-of-sound measuring device 901′ includes two transducers 902 and 905 for wave transmission, and two transducers 903 and 904 for wave reception. The device 901′ transmits an ultrasonic wave from the transducer 902 for wave transmission, and receives a leaky surface wave produced due to the transmission with the two transducers 903 and 904 for wave reception. Further, an ultrasonic wave is similarly transmitted from the transducer 905 for wave transmission, and the two transducers 903 and 904 for wave reception receive a leaky surface wave produced due to the transmission.
The speed of sound of the surface wave can be calculated in consideration of inclination of the bone surface by using a time difference between wave-reception timings of the leaky surface wave by the two transducers 903 and 904 for wave reception when the ultrasonic wave is transmitted from the transducer 902 (or 905) for wave transmission, and by using a difference between a propagation time from a wave transmission to a wave reception when the ultrasonic wave is transmitted from the transducer 902 for wave transmission and the leaky surface wave is received by the transducer 903 (or 904) for wave reception and a propagation time from a wave transmission to a wave reception when the ultrasonic wave is transmitted from the transducer 905 for wave transmission and the leaky surface wave is received by the transducer 903 (or 904) for wave reception.
Further, JP2003-517328(A) discloses, in addition to the speed-of-sound measuring device equipped with two transducers for wave transmission and two transducers for wave reception as the device 901′ described above, a speed-of-sound measuring device including an arrayed transducer including a plurality of transducers. This device first transmits ultrasonic waves from the arrayed transducer to a bone, and then receives reflected waves from a surface of the bone. The device carries out imaging of a shape of the bone front surface by a known method based on the received wave signals, and then derives the thickness of soft tissues.
The device calculates from the thickness of soft tissues an optimum spaced distance between the transducers for wave transmission and the transducers for wave reception. When the soft tissues are thick, if the interval between the transducers for wave transmission and the transducers for wave reception is too near, leaky surface waves cannot be received. On the other hand, when the soft tissues are thin and the interval between both transducers is too far, only leaky surface waves of small amplitude can be received and, therefore, it may not be desirable. Thus, the optimum distance between the transducers for wave transmission and the transducers for wave reception may vary depending on the thickness of soft tissues.
Next, the device determines two transducers for wave transmission and two transducers for wave reception to be used for a speed-of-sound measurement based on the calculated optimum spaced distance between the transducers for wave transmission and the transducers for wave reception among transducers constituting the arrayed transducer. Then, the device calculates a speed of sound of the surface wave using the four determined transducers.
However, the device 901′ or the device disclosed in JP2003-517328(A), WO03/099132, and WO03/099133 calculates a speed of sound based on the propagation course of the ultrasonic wave when the bone surface is flat. Thus, the device can only be applied when the bone surface is flat. Therefore, errors will be greater when a shape of an actual bone surface is curved (for example, when a circumferential speed of sound of a long pipe-shaped bone is measured).
Further, the speed-of-sound measuring device including the arrayed transducer disclosed in JP2003-517328(A) acquires the shape of the bone front surface in an image, using the arrayed transducer. However, this image is only originally used to detect the thickness of soft tissues and determine four transducers to be used for a speed-of-sound measurement among the arrayed transducers based on the thickness and, thus, it is not for calculation of a speed of sound.